chapter 7 three point test cross
region 2 map distances
# SCO + DCO / total progeny * 100
the amount of interference varies
- from one chromosome to another - from one part of the chromosome to another - with gene location on the chromosome arm - with species - when genes regulate crossovers
steps in analyzing data from a three-point test cross
1. identify the parents (most frequent class) 2. identify the double crossovers (least frequent class) 3.determine gene order (which gene is in the middle) 4. re-write the gametes with the correct gene order 5.determine products of crossovers in each of region 1 and region 2 6. determine recombination frequencies in each of region 1 and region 2 7. map the genes
trihybrid testcross
8 different classes are in this cross
region 1 map distances
= # SCO + DCO / total progeny * 100
region 1 map distances
= % recombinants between p and j
coefficient of coincidence
= observed frequency of double crossovers / expected frequency of double crossovers
region 2 map distances
=% recombinants between j and r
decreases
Accuracy of our genetic map _________ if (1) genes are far apart and (2) there are double crossovers
to determine the origin
__________- of the other two recombinant classes, we need to look at the effect of crossovers between pairs of genes arranged in the correct order
genetic map distance
_______________ equals recombination frequency if genes are close together and there are no multiple crossovers between the genes
lack of independence
a ______________ in these two events (region 1 and 2 crossovers). once a crossover occurs in region 1 the probability of a crossover in region 2 is reduced (or vice versa) this is probably due to physical interference cause by breaking and redoing the chromatids
3 point test cross
a test cross involving three linked genes in a small region of the chromosome
3 point test cross
a test cross of a trihybrid
to get better maps (more accurate map)
a) study genes that are closer together b) use a 3-point test cross (trihybrid test cross) c) both a and b increase the accuracy of the genetic map
DCO
always the least frequent
parental phenotypes
always the most frequent
double crossover
for our map the probability of a ___________ is 0.208 * 0.10 = 0.0208 or 2.08% -this would be the expected number of __________
there are no multiple crossovers genetic distances between genes are small
genetic map distance = recombination frequency when
genes are far apart on the chromosome or there are multiple crossovers between genes
genetic map distance does not equal recombination if
independent
if a crossover is __________ the probability of a double crossover is the product of the probabilities of the two ____________ events (product rule)
independent
if double crossovers are _____________ we would expect 2.08%
interference
is measured by the coefficient of coincidence
interference
is when we observe fewer double crossovers than we expected based on the map distances between two genes
DCO
means double crossover
SCO
means single crossover
double crossover
occurs in both region 1 and region 2 and therefore is added into the recombination frequencies for the regions
two strand double crossover
problem occurs when the genes are far enough apart that more than one cross-over occurs between them
double crossover
recombinant chromosomes resulting from the _________ have only the middle gene altered
interference
reduces DCOs - when region 1 or region 2 prevents the other from happening.
region 2 map distances
the distance between j and r
region 1 map distances
the distance between p and j
with a third gene in the middle and a double crossover
the middle allelic pair changes orientation relative to the outside allelic pairs
3 point test cross
used to determine the recombination frequencies among the 3 genes and therefore, the relative distances between these genes.
3 point test cross
used to map genes to linkage groups
trihyrbid (a+ c+ b+/ a c b)
when mapping genes in a _________ the gametes in which one gene changes its orientation relative to the other two tells you which of the 3 genes in the ________ is in the middle